Glutamate is the main excitatory neurotransmitter in the central nervous system (CNS) and mediates neurotransmission across most excitatory synapses. Three classes of glutamate-gated ion channel receptors (N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and Kainate) transduce the postsynaptic signal. Of these, NMDA receptors (NMDAR) are responsible for a significant portion of the excitotoxicity of glutamate. NMDA receptors are complex having an NR1 subunit and one or more NR2 subunits (2A, 2B, 2C or 2D) (see, e.g., McDain, C. and Caner, M. (1994) Physiol. Rev. 74:723-760), and less commonly, an NR3 subunit (Chatterton et al. (2002) Nature 415:793-798). The NR1 subunits have been shown to bind glycine, whereas NR2 subunits bind glutamate. Both glycine and glutamate binding are required to open the ion channel and allow calcium entry into the cell. The four NR2 receptor subunits appear to determine the pharmacology and properties of NMDA receptors, with further contributions from alternative splicing of the NR1 subunit (Kornau et al. (1995) Science 269:1737-40). Whereas NR1 and NR2A subunits are ubiquitously expressed in the brain, NR2B expression is restricted to the forebrain, NR2C to the cerebellum, and NR2D is rare compared to the other types.
Because of the key role of NMDA receptors in the excitotoxicity response, they have been considered as targets for therapeutics. Compounds have been developed that target the ion channel (ketamine, phencyclidine, PCP, MK801, amantadine), the outer channel (magnesium), the glycine binding site on NR1 subunits, the glutamate binding site on NR2 subunits, and specific sites on NR2 subunits (Zinc—NR2A; Ifenprodil, Traxoprodil—NR2B). Among these, the non-selective antagonists of NMDA receptor have been the most neuroprotective agents in animal models of stroke. However, clinical trials with these drugs in stroke and traumatic brain injury have so far failed, generally as a result of severe side effects such as hallucination and even coma.
Postsynaptic density-95 protein (PSD-95) couples NMDARs to pathways mediating excitotoxicity and ischemic brain damage (Aarts et al., Science 298, 846-850 (2002)). This coupling was disrupted by transducing neurons with peptides that bind to modular domains on either side of the PSD-95/NMDAR interaction complex. This treatment attenuated downstream NMDAR signaling without blocking NMDAR activity, protected cultured cortical neurons from excitotoxic insults and reduced cerebral infarction volume in rats subjected to transient focal cerebral ischemia. Treatment was also reported to be protective in a rat model of fluid percussion injury in which a fluid volume is rapidly injected into the cranial cavity (see US 20050059597). The injury in such a model has a mild primary effect in terms of immediate tissue damage but extensive secondary effects results from excitoxicity.